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  • 11
    Publication Date: 2021-03-29
    Description: The presented thesis describes the observations of the Galactic center Quintuplet cluster, the spectral analysis of the cluster Wolf-Rayet stars of the nitrogen sequence to determine their fundamental stellar parameters, and discusses the obtained results in a general context.The Quintuplet cluster was discovered in one of the first infrared surveys of the Galactic center region (Okuda et al. 1987, 1989) and was observed for this project with the ESO-VLT near-infrared integral field instrument SINFONI-SPIFFI. The subsequent data reduction was performed in parts with a self-written pipeline to obtain flux-calibrated spectra of all objects detected in the imaged field of view ...
    Description: thesis
    Keywords: 523 ; TIC 000 ; THT 800 ; TIE 662 ; Sternhaufen {Astronomie} ; Massenreiche Sterne {Astronomie} ; Struktur der Milchstrasse {Astronomie}
    Language: German
    Type: monograph , publishedVersion
    Format: 115 S.
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  • 12
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    Univ. Bochum
    Publication Date: 2020-10-07
    Description: Die Dissertation behandelt die Konzepte sowie die Implementierung der operativen Aspekte der Kontrollsoftware des LUCIFER Instruments. Diese ist in vier Schichten aufgeteilt. Die Schnittstelle zwischen Software und Hardware bildet die Control-Schicht. In der Instrument-Schicht sind alle Komponenten des Instruments als Software-Dienste abgebildet und erfüllen die jeweils für diese Komponente notwendigen Aufgaben. In der obersten Schicht sind die wesentlichen Manager-Dienste beheimatet. Durch Beobachtungsskripte ist eine automatische Steuerung von Instrument und Teleskop möglich. Weiterhin wurden die stellaren Populationen im Zentrum von vier Galaxien untersucht. Ein Vergleich mit publizierten Werten für ähnliche Galaxien zeigt keinen signifikanten Trend zu jüngeren Populationen in den Galaxien, die einen Pseudo-Bulge zeigen. Eine Galaxie zeigt deutliche Hinweise auf eine sehr alte stellare Population im Buge, deutlich älter als die Populationen von elliptischen Galaxien.
    Description: thesis
    Keywords: 520 ; TCM 000 ; TIE 400 ; Mathematische und EDV-Verfahren {Astronomie} ; Spektren und Leuchtkraft von Sternsystemen {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 142 S.
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  • 13
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    Univ. Freiburg (Breisgau)
    Publication Date: 2021-03-29
    Description: Observations of the solar corona show loop-like structures formed by plasma at temperatures of one million degrees and higher. Since the solar surface is much cooler than the corona, a heating mechanism must be responsible for the high temperatures. The dissipation of magnetic fields in the corona could provide such a heating mechanism. However, the process of transforming magnetic energy into thermal energy is still not yet understood in detail. To investigate this process and its impact on the heating of the corona, we employ a three-dimensional magneto-hydrodynamical model. This numerical model synthesizes the temporal evolution of the magnetic field above an Active Region. It includes the solar atmosphere from the photosphere up to the corona. The magnetic field in the corona is braided by foot point motions in the photosphere. This is done imilarly to the braiding through granulation in reality. The stressed agnetic field induces currents which are then dissipated in the corona. This dea is known as the DC model (direct current) and was proposed by Parker in 1972. The model reaches a quasi-stationary state, i.e. the energy input by hotospheric motions is counterbalanced by radiative losses in the optically thin corona. As a result, the described heating process creates and sustains a hot corona with a temperature of one million degrees and higher ...
    Description: thesis
    Keywords: 523 ; TGC 765 ; TGC 800 ; Korona {Astronomie: Sonne} ; Magnetfelder {Astronomie: Sonne}
    Language: English
    Type: monograph , publishedVersion
    Format: 108 S.
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  • 14
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    Univ. Potsdam
    Publication Date: 2021-03-29
    Description: Stellar magnetic fields, as a crucial component of star formation and evolution, evade direct observation at least with current and near future instruments. However investigating whether magnetic fields are generated by a dynamo process or represent relics from the formation process, or whether they show a behavior similar to the sun or something very different, it is essential to investigate their structure and temporal evolution. Fortunately nature provides us with the possibility to indirectly observe surface topologies on distant stars by means of Doppler shift and polarization of light, though not without its challenges. Based on the mentioned effects, the so called Zeeman-Doppler Imaging technique is a powerful method to retrieve magnetic fields from rapid rotating stars based on measurements of spectropolarimetric observations in terms of Stokes profiles. In recent years, a large number of stellar magnetic field distributions could be reconstructed by Zeeman-Doppler Imaging (ZDI). However, the implementation of this method often relies on many approximations because, as an inversion method, it entails enormous computational requirements.
    Description: thesis
    Keywords: 523 ; THR 000 ; TEG 000 ; Elektrische und magnetische Felder {Sterne} ; Kosmogonie {Astronomie}
    Language: German
    Type: monograph , publishedVersion
    Format: 107 S.
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  • 15
    Publication Date: 2021-03-29
    Description: This work focuses on the distribution of atomic hydrogen in the exosphere of Saturns̉ moon Titan. This work is based on measurements performed by the Hydrogen Deuterium Absorption Cell (HDAC) aboard the Cassini spacecraft, that should directly determine the D/H ratio from the UV emission of atomic hydrogen and deuterium at 121.567 and 121.533 nm, respectively. HDAC measurements were performed on December 26, 2005 during the ninth Titan flyby of the Cassini orbiter. Unfortunately the amount of deuterium in the absorption cell could not be determined. Thus, the purpose, HDAC was designed for could not be achieved. For this reason, HDAC has performed measurements only once. However, the amount of atomic hydrogen in the absorption cell is well known, hence the data using only the hydrogen cell can be used in order to investigate Titans̉ exosphere. This work aims at simulating the HDAC measurements performed in order to investigate Titan’s exosphere, e.g. to determine the distribution of atomic hydrogen. A model of Titan’s exosphere including atomic hydrogen and methane has been developed and the transfer of solar radiation is calculated in order to simulate the HDAC measurements. The radiative transfer is solved using the Monte Carlo method. For the exospheric model, two different atomic hydrogen density distributions were applied, which determine the distribution in the exosphere by different approaches. Density profiles calculated by both models mainly differ in the lower exosphere ...
    Description: thesis
    Keywords: 523 ; TGG 655 ; Saturnmonde {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 171 S.
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  • 16
    Publication Date: 2020-10-07
    Description: The first part of the thesis is concerned with the fate of singularities in quantum cosmology. The second part addresses the derivation of predictions from quantum cosmology. In the first part, two classes of cosmological models were studied. In the first class of models, the universe evolves to or emerges from a big-rip singularity. Here, energy density, pressure and scale factor diverge after a finite amount of time. This type of singularity arises rather generically in cosmological models with phantom dark energy. For each of these phantom-field models, the corresponding scenario with ordinary scalar field was studied. The scalar field induced a big-bang singularity. The second class of models studied was dominated by a big-brake singularity. At the big brake, the universe evolution comes to a halt due to an infinite deceleration. The motivation behind this choice of models was the occurrence of a singularity at large scale factor. The major question pursued was whether these types of singularity were resolved on the quantum level. If such singularities were resolved in quantum cosmology, this would imply that quantum gravitational effects can occur in the macroscopic universe. After devising classical models that contain the respective singularity, I subjected these models to quantization which was carried out in the geometrodynamical approach. The governing equation is then the WheelerDeWitt equation ...
    Description: Zsfassung in dt. und engl. Sprache
    Description: thesis
    Keywords: 523 ; TEC 800 ; Quantenkosmologie {Astronomie: Kosmologie}
    Language: English
    Type: monograph , publishedVersion
    Format: 228 S.
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  • 17
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    Univ. Zugl.: Braunschweig
    Publication Date: 2021-03-29
    Description: Seit den 60er Jahren des letzten Jahrhunderts ist Mars immer wieder das Ziel von Raumfahrtmissionen gewesen, um unter anderem nach Wasser auf unserem Nachbarplaneten zu suchen. Nachdem die bisher analysierten Oberflächenmerkmale auf ein Vorhandensein von flüssigem Wasser beim Entstehen der Oberfläche deuten, wurde im Juni 2008 von der Sonde Phoenix erstmals Wassereis unter der Oberfläche direkt nachgewiesen. In dieser Arbeit wird untersucht, wieviel Wasser Mars in den vergangenen 4,5 Milliarden Jahren verloren hat und welche Mechanismen dabei eine Rolle gespielt haben. Bei den nichtthermischen Mechanismen hat insbesondere die Umströmung des Sonnenwindplasmas um die Marsatmosphäre mittels seiner elektromagnetischen Felder entscheidenden Einfluß auf den Verlust planetarer Sauerstoff- und Wasserstoffionen. Dieses Wechselwirkungsszenario wurde mit numerischen Plasmasimulationen untersucht. Das verwendete Hybrid-Modell erfasst vollständig die Ionendynamik durch eine kinetische Beschreibung der Ionen. Die Elektronen werden als massenlose Flüssigkeit modeliert. Eine Reihe von Ergebnissen wird vorgestellt: (a) In Übereinstimmung mit dem ASPERA-3 Experiment auf Mars-Express werden zwei Plasmagrenzschichten und zwar der Bow Shock und die "Ion Composition Boundary" identifiziert. (b) Es wird gezeigt, dass beim Vorbeiflug der Raumsonde Rosetta eine Anomalie des Oberflächenmagnetfeldes registriert wurde ...
    Description: thesis
    Keywords: 523 ; TGC 900 ; TGG 565 ; Solar-terrestrische Erscheinungen {Astronomie} ; Marsatmosphäre {Astronomie}
    Language: German
    Type: monograph , publishedVersion
    Format: 193 S.
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  • 18
    Publication Date: 2021-03-29
    Description: After the epoch of reionisation the intergalactic medium (IGM) is kept at a high photoionisation level by the cosmic UV background radiation field. Primarily composed of the integrated contribution of quasars and young star forming galaxies, its intensity is subject to spatial and temporal fluctuations. In particular in the vicinity of luminous quasars, the UV radiation intensity grows by several orders of magnitude. Due to an enhanced UV radiation up to a few Mpc from the quasar, the ionised hydrogen fraction significantly increases and becomes visible as a reduced level of absorption in the HI Lyman alpha (Ly-alpha) forest. This phenomenon is known as the proximity effect and it is the main focus of this thesis ...
    Description: thesis
    Keywords: 523 ; TBI 000 ; Ultraviolett-Astronomie
    Language: German
    Type: monograph , publishedVersion
    Format: 94 S.
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  • 19
    Publication Date: 2021-03-29
    Description: Spectro-astrometry is a technique to obtain information about the structure of an astronomical source on sub-diffraction limited spatial scales down to milli- or micro-arcseconds. The method exploits the shift of the photocentre of a structured source over emission/absorption lines in its spectral energy distribution. Sub-diffraction limited resolution can be achieved because the method only uses wavelength-differential information of longslit spectra. Although spectro-astrometry has already been devised by Bailey (1998) and some studies have been performed to date on different astronomic targets, so far, no systematic analysis of the potential of this technique has been undertaken. The aim of this work was to analyse the potential of the method, develop the tools to exploit this potential and write the software which is needed to perform the analysis procedure. To verify the practical feasibility, observations were done with the VLT/CRIRES facility to demonstrate the capabilities of the method with respect to detecting structures on giant star surfaces. All steps of data reduction were analysed with respect to their influence on the spectro-astrometric quantities and a new method for bad pixel detection was devised and implemented ...
    Description: thesis
    Keywords: 523 ; THT 600 ; THK 000 ; THT 400 ; Riesensterne {Astronomie} ; Spektren und Spektroskopie von Sternen {Astronomie} ; Kalte Sterne {Astronomie}
    Language: English
    Type: monograph , publishedVersion
    Format: 109 S.
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  • 20
    Publication Date: 2021-03-29
    Description: The CERN Axion Solar Telescope (CAST) is searching for solar axions, which could be produced in the core of the Sun via the so-called Primakoff effect. For this purpose, CAST uses a decommissioned LHC prototype magnet. In its magnetic field of 9 Tesla axions could be reconverted into X-ray photons. The magnet is mounted on a structure built to follow the Sun during sunrise and sunset for a total of about 3 hours per day. The analysis of the data acquired during the first phase of the experiment with vacuum in the magnetic field region yielded the best experimental upper limit on the axion-to-photon coupling constant for axion masses up to about 0.02 eV. In order to extend the sensitivity of the experiment to a wider mass range, the CAST experiment continued its search for axions with helium in the magnet bores. In this way it is possible to restore coherence for larger masses. Changing the pressure of the helium gas enables the experiment to scan different axion masses. In the first part of this second phase of CAST, helium-4 has been used and the axion mass region was extended up to 0.4 eV ...
    Description: thesis
    Keywords: 523 ; TII 100 ; TCE 380 ; Dunkle Materie {Astronomie: Interstellare Materie} ; CCD, Photodetektoren {Astronomische Instrumente}
    Language: English
    Type: monograph , publishedVersion
    Format: 266 S.
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